Crazy "Batman" Winch Question...

Joined
Feb 4, 2016
Messages
2
Hey Everyone,

I'm a noob when it comes to high power brushless stuff, so forgive me if this isn't an appropriate board for this question. I've got a project where the math works out, but I don't have any hands-on experience and was hoping to get some opinions.

Rather than pulling a glider, I'd like to make a winch that can pull me straight up 20-30 feet. I'd like to keep it as small and cheap as possible, and there seems to be a pretty steep price difference between a 6S ESC and a 12S ESC, so:

-A 240 Kv motor like this one http://www.hobbyking.com/hobbyking/...0_High_Voltage_Brushless_Outrunner_240KV.html
has a Kt of (240/(60/2pi))^-1 = 0.0398. At 22 Volts that gives a motor speed of 552.92 rad/s

-With a 12:90 tooth belt from motor to reel, the reel speed will be 552.92/7.5 = 73.72 rad/s. Assuming the reel diameter is 1/2" (0.00635m radius) that becomes a line speed of ~0.47 m/s, so ascending about 1.5 feet per second top speed.

-Assuming 100kg of weight being pulled up, with 1 m/s^2 acceleration, that requires 1081 N, or 6.86 Nm of torque from the reel. Knowing that T = I * Kt * gear ratio that comes out to 22.9 amps of current draw. The rope is a 1/8" diameter nylon paracord, and as that wraps around the reel, the reel diameter is going to effectively increase. That hopefully tops out at a reel diameter of 1", or .0127m radius. At that point the torque required will be doubled, so 13.72 = I * 0.0389 * 7.5, so I then = 47.03 amps.

This seems manageable number wise, well within the motor specs, a 6S 100 amp ESC and a 22.2 3000 mAh LiPo with 35C ought to be able to handle the NUMBERS. I have a BS in Electrical Engineering, but I've never so much as held a brushless outrunner before, so I have no idea if this makes PHYSICAL sense.

TL;DR In your opinion, does a 240 Kv motor on 6S with a 1:7.5 gear reduction have enough oomph to reel 200lbs straight up at 1.5 feet/sec? What would you recommend otherwise?
 
Your thinking looks ok to me and it comes down to whether that motor will do put out 1-2Nm for ~30 seconds without overheating. I don't have experience with that motor, but I'd think you stand a fair chance.

Something I think you may have missed is the effect of load on the motor speed. Sure, 240Kv x 22V is 553 Rad/sec but that's at no-load. The easiest solution would be to read over a power curve for the motor to see what speed you'd have at 1 or 2Nm. You might be able to derive it from the motor data but I'm not sure you have enough available.
 
Thanks for replying, Punx0r! The loaded motor speed is definitely something I'm in the dark about, I'll be sure to look around for motors that have their power curve available. I don't need to use the specific motor I linked to, I just wanted an example as a starting point. It's definitely in my favor that I don't plan to have the motor running for more than 30 seconds as you said, so I feel like I might be able to get away with more abuse.
 
Dunno if you are still checking in on this thread but.... A few things to consider (myself having built a brushless winch for rc gliders):
1) The ESC will blow up if you try to start motor with a significant tension on the rope.
2) Not sure what you have in mind after the 30' ride up, but the motor alone probably wont provide enough braking to keep you from falling back down.
3) Your scheme will result in double speed (3 ft/sec) at end of run. Hope thats OK for your plans.
4) If you plan part throttle operation, esc may be too small and you will need to add capicitors to input leads. I.e., 50 motor amps at 25% throttle reequires 50/.25 = 200 amp esc and tons of input caps.
5) If the system is stiff (short length of cord with little flex, no shock aborbtion mechanism, etc) you will probably get pretty considerable current spikes as your body bobs up and down on the rope. May or may not blow things up.
 
I'm not trying to understand what you are trying to accomplish, but did you see this?

I believe their first iteration was crotch based.

[youtube]kX3mPHsdN0k[/youtube]
 
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